“There’s a subgroup population of kids who don’t seem to be experiencing the same benefit of positive movement as others,” said Dr. Lauren Smith.

After a three-decade tripling in childhood obesity rates, the trend has leveled off and, for the first time, appears to be on a substantial decline - at least among Massachusetts infants and preschoolers, according to a study published Monday in the journal Pediatrics.

Researchers at the Harvard Pilgrim Health Care Institute found that the percentage of obese girls under age 6 dropped from 9 percent to slightly more than 6 percent from 2004 to 2008; the percentage of obese boys under age 6 fell from nearly 11 percent to just under 9 percent during the same time period.

“This is a surprising and encouraging finding,’’ said study leader Dr. Xiaozhong Wen, a postdoctoral fellow at Harvard Pilgrim.

The study analyzed electronic medical records of nearly 37,000 children from birth to age 5 in Harvard Vanguard Medical Associates’ pediatric practice group in Eastern Massachusetts. The positive results could indicate that increasing public awareness about the dangers of childhood obesity and state health programs to prevent it could finally be paying off.

The next step, Wen said, is “to figure out what led to this decline and whether it’s a national trend.’’

A 2010 national government survey detected a slight decline in obesity rates among preschoolers in recent years but didn’t contain a large enough sample of young children to determine whether it was a true statistical trend. Another national study did not find any drop in rates but that could be because it included only youngsters from low-income families who have traditionally higher obesity rates.

The latest study found the same glaring discrepancy between those from high-income and low-income families: Children on Medicaid - coverage provided to those near the poverty level - had obesity rates that dropped on average from 12.3 percent to 11.5 percent, a change small enough to be due to statistical chance. Those covered by other health plans had a more significant decline in obesity rates from 10.1 percent to 8.3 percent.

“There’s a subgroup population of kids who don’t seem to be experiencing the same benefit of positive movement as others,’’ said Dr. Lauren Smith, the state public health department’s medical director. “We have to always be cognizant and cautious that interventions that we’re doing are equitably distributed across all populations at risk so we don’t exacerbate disparities.’’

Previous state data has shown “great disparities in obesity prevalence throughout different Massachusetts communities,’’ Michael Leidig, clinical director of the Center for Youth Wellness at Floating Hospital for Children at Tufts Medical Center, pointed out in an e-mail. For example, higher-income communities such as North Andover and Brookline have obesity rates in the 8 to 10 percent range for school-age children compared with rates of about 25 percent for their respective neighboring lower-income communities of Lawrence and Boston.

A lack of access to affordable fresh fruits and vegetables in lower-income communities and safe places for children to play outdoors, said Smith, could help explain these differences.

Where the state has made progress is in health initiatives to increase breastfeeding rates and reduce smoking during pregnancy, two factors that the Harvard Pilgrim study authors speculated contributed to the overall decline in obesity rates during early childhood.

According to state health data, just under 7 percent of babies born in 2008 were to mothers who smoked during pregnancy compared with 19 percent born in 1990; evidence suggests that babies born to smoking mothers have a greater risk of obesity possibly because exposure to cigarette smoke in the womb interferes with the genetic programming of appetite regulation in fetuses. Breastfed babies have a lower likelihood of becoming overweight by age 3 compared with formula-fed infants, possibly because they are better able to regulate how much they consume.

One of the main reasons for the decline, however, could simply be due to parents, doctors, and preschools taking determined action to prevent excessive weight gain at a point when they can exert considerable control over what children eat, that is, before they start grabbing meals and snacks on their own.

“I’ve seen some increased awareness in day-care centers and efforts to introduce healthier foods such as fruits, cut-up vegetables, and cheese instead of crackers and cookies,’’ said Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. “And the message may be getting out about the importance of limiting juice and sugary drinks.’’

In an effort to limit excess calorie consumption, the American Academy of Pediatrics began recommending several years ago that children ages 1 to 6 drink no more than 4 to 6 ounces of fruit juice each day. And the federal government’s Special Supplemental Nutrition Program for Women, Infants, and Children, or WIC, food package program now mandates reduced juice subsidization for children under 5.

Karen Yates, a mother of two from Malden, said her 6-year-old son learned about good nutritional habits in preschool last year. “They did a whole unit on identifying ‘go, slow, and whoa’ foods,’’ she said, referring to a school nutrition program developed at the University of California, “and he really did learn a lot about the kinds of foods he should eat and what he should limit.’’ She said her 3-year-old won’t see candy or treats at school birthday parties since they are banned.

Unfortunately, specialists caution, the latest obesity data does not mean that we can close the book on the nation’s weight problems. “We have to treat the gains that we’re seeing as fragile and not certain,’’ said Dr. James Marks, vice president of the nonprofit Robert Wood Johnson Foundation Health Group in Princeton, N.J. “We might have turned the corner but the epidemic isn’t over. We have a long way to go to get back to where we were in the 1970s.’’